Preparation of a lead sensor based on porous multiwalled carbon nanotubes/thiolated chitosan composite materials

Thiolated chitosan (CS–TGA) was prepared using chitosan (CS) and thioglycolic acid (TGA). Then MWCNTs were added to the mixture of CS–TGA and CS to prepare the CS/CS–TGA/MWCNs porous composite by freeze-drying method and this composite was used to modify an indium tin oxide glass electrode. The electrode was used as a sensor for Pb2+. The morphology and structure of the composite were characterized by infrared spectroscopy and scanning electron microscope, and their electrochemical behavior was also studied.

⃝ T¨UB˙ITAK

doi:10.3906/kim-1212-33

h t t p : / / j o u r n a l s t u b i t a k g o v t r / c h e m /

Research Article

Preparation of a lead sensor based on porous multiwalled carbon

nanotubes/thiolated chitosan composite materials

Jun WAN∗, Ling XING, Wei WANG

College of Environment and Safety Engineering, Key Laboratory of Eco-chemical Engineering,

Ministry of Education, Qingdao University of Science and Technology, Qingdao, P.R China

Abstract:Thiolated chitosan (CS–TGA) was prepared using chitosan (CS) and thioglycolic acid (TGA) Then MWCNTs

were added to the mixture of CS–TGA and CS to prepare the CS/CS–TGA/MWCNs porous composite by freeze-drying method and this composite was used to modify an indium tin oxide glass electrode The electrode was used as a sensor for Pb2+ The morphology and structure of the composite were characterized by infrared spectroscopy and scanning electron microscope, and their electrochemical behavior was also studied Under the optimized experimental conditions, the sensor showed a linear range of 2.0× 10 −9 ∼2.0 × 10 −8 mol L−1 for Pb2+ with a detection limit of 9.53 × 10 −10

mol L−1 according to the 3 σ rule The prepared heavy ion sensor displayed excellent electrochemical response and high

sensitivity

Key words: Thiolated chitosan, composite materials, porous materials, lead sensor

1 Introduction

Chitosan is a kind of renewable natural biopolymer, and its amino and hydroxyl can easily form high charge density cationic polyelectrolyte in acidic solution, which means it has good complexation and adsorption properties In order to extend the scope of the application of chitosan and make it easy to separate and regenerate, cross-linking, grafting, acylation, and etherification were introduced to improve the nature of the

effectively improve the adhesion, permeability, and swelling behavior of chitosan As a result, heavy metal ions can be firmly absorbed on the chitosan

Due to their possessing unique electronic, mechanical, structural properties, and energy storage, carbon nanotubes (CNTs) have greatly attracted the attention of researchers all over the world since they were found

∗Correspondence: wanjundz@sohu.com

Lead and its compounds are toxic chemicals in the list of environmental pollutants They cannot be degraded in water and do great harm to the environment and life Even at extremely low concentrations, lead can cause nerve dysfunction, kidney damage, and reproductive system damage Due to the great concern about lead contamination, there is still an urgent demand for lead ion detection techniques There are many

modified carbon paste electrode exhibited well-defined and separate stripping voltammetric peaks for cadmium

nanomaterial/ionophore on a glassy carbon electrode, using nanosized hydroxyapatite to improve the sensitivity and establishing a sensitive electrochemical method of determination of lead, with a linear range of 5.0 nM to

In this paper, chitosan/chitosan–thioglycolic acid/multiwalled carbon nanotube composites were prepared

and sensitive for the detection of heavy metal ions in a certain concentration range

2 Experimental

2.1 Reagents and instruments

dimethyllaminopropyl) carbodiimide hydrochloride (EDC), N–hydroxysuccinimide (NHS), and other reagents were of analytical grade and used as received without further purification All solutions were prepared with doubly distilled water All the electrochemical measurements were carried out on a CHI 832B electrochemical analyzer (Shanghai Chen Hua Instrument Co Ltd.) A platinum wire and an Ag/AgCl electrode were used

as auxiliary electrode and reference electrode, respectively An ITO glass electrode and modified ITO glass electrodes were used as working electrode The as-prepared samples were analyzed by JSM-6700F scanning electron microscopy (SEM, Japan Electron, Japan) and Nicolet FT-IR 510P spectrophotometer (IR, Nicolet, USA)

2.2 Preparation of CS/CS–TGA/MWCNTs composite

Appropriate amounts of EDC and NHS were added to CS–HCl solution under stirring Then thioglycolic acids were added to the solution to adjust pH to 5 After stirring for 3 h at room temperature, the reaction solutions were removed into a dialysis bag in order to obtain the thiolated chitosan The prepared thiolated chitosan was

2.3 Preparation of CS/CS–TGA/MWCNTs/ITO electrode

A glass electrode was cleaned in ethanol in an ultrasound cleaner for 5 min Then 10 µ L of the prepared

composite–acetic acid solution was deposited directly onto the ITO electrode The electrode was then quickly

2.4 Electrochemical determination of Pb2+ on the modified electrode

Before each test, the modified electrode was immersed into the solution for adsorption under stirring After enriching for a certain period of time, the electrode was removed from the solution and washed using ultrapure water Then the electrode was placed in the electrolytic cell containing the supporting electrolyte, and was

3 Results and discussion

3.1 The IR characterization of CS and CS–TGA

Figure 1 shows the infrared spectra of CS–TGA and CS Compared with the infrared spectrum of CS, a –SH

characteristic frequency in organic sulfides, indicating that the as-prepared compound contained –SH functional groups

4 0 0 0 3 0 0 0 2 0 0 0 1 0 0 0

0 4 5

0 5 0

0 5 5

0 6 0

0 6 5

0 7 0

0 7 5

0 8 0

0 8 5

0 9 0

0 9 5

1 0 0

1 0 5

3 4 3 8 9 4 1 6 6

2 4 5 9 1 4 2 2 4

1 6 2 3 9 9 8 2 5

1 0 8 9 7 3 7 5 4

C S - T G A

C S

Figure 1 IR-spectra of CS–TGA and CS.

3.2 Characterization of the CS/CS–TGA/MWCNTs composite

In this work, porous CS/CS–TGA/MWCNTs composite scaffolds were fabricated from aqueous mixtures of chitosan/thiolated chitosan and MWCNTs using a 2-step process in which the CS/CS–TGA/MWCNTs solution

selected to be 7:3 The content of MWCNTs in the composites was optimized The resulting scaffold porous structure with different mass fractions of MWCNTs was characterized by SEM images (Figures 2a–2d) As the concentration of MWCNTs in the precipitate increases, the degree to which the tips of the MWCNTs puncture the precipitate’s surface increases At the lowest MWCNT concentrations (the content of MWCNTs was about 0.83 wt%, Figure 2a), the CS/CS–TGA surface was mostly smooth with few distinguishable MWCNTs visible, and the pore size distribution was not uniform (see Figure 2a) At higher MWCNT concentrations (the MWCNTs content was 3.5 wt% and 5.0 wt%, Figures 2c and 2d), the tips of the MWCNTs began to puncture the surface, the pore size was too large, and the distribution was uneven and not suitable for absorption (see Figures 2c and 2d) When the MWCNTs content was 2.5 wt% (Figure 2b), the pore size distribution was relatively uniform, and the pore size was suitable for absorption The amount of 2.5wt% MWCNTs was selected in the next experiment

Figure 2 SEM image of CS/CS–TGA/MWCNTs composite material with different content of MWCNTs: (a) the

content of MWCNTs is 0.83 wt%; (b) the content of MWCNTs is 2.5 wt%; (c) the content of MWCNTs is 3.5 wt%; (d) the content of MWCNTs is 5.0 wt%

3.3 Optimization of the Pb2+ determination conditions

The effect of pH value of the substrate solution on the electrochemistry was examined, and the results are shown in Figure 3a It can be seen that the peak current increased as the pH increased from 3.5 to 5.0 and the maximum response was at pH 5.0; then it decreased Thus, the optional pH value was 5.0 and was chosen for all experiments Figure 3b was the adsorption time optimization curve of the electrode soaked in the solution

peak current increased with adsorption time, and reached its maximum when the adsorption time was 35 min;

it then decreased slowly This was probably because of the electrode adsorption reaching equilibrium at 35 min

We chose the adsorption time of 35 min as the optimal time

3.5 4.0 4.5 5.0 5.5 6.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Potential/V

a

1 2 3 4 5 6

T/min b

Figure 3 Optimization conditions of (a) pH and (b) adsorption time.

3.4 Amperometric response to Pb2+

water (0.01 mg/L) according to the World Health Organization (WHO) The modified electrodes display better

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

Potential/V

a g

2.0 2.5 3.0 3.5 4.0 4.5 5.0

Concentration/10-9M

Figure 4 a) SWV curves obtained after immerging in different concentration of Pb2+ (a) 2 × 10 −9; (b) 4 × 10 −9;

(c) 6 × 10 −9; (d) 8 × 10 −9; (e) 1 × 10 −8; (f) 1.2 × 10 −8; (g) 1.4 × 10 −8 in 0.1 mol L−1 acetate buffer solution

(pH 5.0) of CS/CS–TGA/MWCNs/ITO electrode; b) Calibration plot of peak current versus Pb2+ concentration (the detection condition was similar to a.)

3.5 Reproducibility and interference

sensor was excellent

selectivity of the sensor is quite good

0 1 2 3 4 5

Pb2+ Pb2+，Cu2+ Pb2+，Cd 2+

Pb2+, Hg2+

Figure 5 The current response of the sensor in 1.0 × 10 −8 mol L−1 Pb2+

solution and Pb2+ with 1.0 × 10 −6 mol

L−1 other different ions solutions

To demonstrate the performance of the developed method, a comparison of the linear range and detection

by other research groups, our results show that the detection limit is good

Table Comparison of some properties in the present work with those in other studies.

OMC, ordered mesoporous carbon; IL2, 1-ethyl-3-methylimidazolium tetrafluoroborate; CILE, carbon ionic-liquid elec-trode; 5-Br-PADAP, 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol

Furthermore, the reproducibility of the electrode was also tested The catalytic current response could maintain about 95% of its original response over 2 weeks when the modified electrode was stored and measured intermittently

4 Conclusion

In this work, the composite of CS/CS–TGA/MWCNs was prepared and was used to modify a glass ITO electrode to form a heavy metal ions sensor The prepared material had good adsorption properties for heavy metal ions and was suitable for a sensor Under the optimal conditions, the sensor showed a linear range of 2.0

× 10 −9 ∼2.0 × 10 −8 mol L−1 for Pb2+ with a detection limit of 9.53 × 10 −10 mol L−1 according to the 3 σ

rule The sensor exhibited high sensitivity and good response to detect heavy metal ions

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No 21175077 and 21105053), the Scientific and Technical Development Project of Qingdao (12-1-4-3-(4)-jch and 11-2-4-3-(8)-jch), and the Nature Science Foundation of Shandong Province (ZR2010BM025)

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